In the recovery of oil from oil-containing formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. These techniques include thermal recovery methods, waterflooding and miscible flooding.
More recently, carbon dioxide has been used successfully as a miscible oil recovery agent. Carbon dioxide is a particularly desirable material because it is highly soluble in oil and dissolution of carbon dioxide in oil causes a reduction in the viscosity of the oil and increases the volume of oil all of which improve the recovery efficiency of the process. Carbon dioxide is sometimes employed under non-miscible conditions and in certain reservoirs it is possible to achieve a condition of miscibility at reservoir temperature and pressure between essentially pure carbon dioxide and the reservoir oil.
U.S. Pat. Nos. 2,875,832 and 3,995,693 disclose the use of CO.sub.2 -hydrocarbon gas mixtures in the recovery of oil from subterranean oil-containing formations wherein produced gas from the formation containing CO.sub.2 and hydrocarbons in recycled to the formation.
The present invention provides a method for recovering oil from subterranean, viscous oil-containing formations wherein carbon dioxide is injected into the formation at pressures at or above the miscible displacement pressure to miscibly displace the oil and hydrocarbon gas to a recovery well. The improvement comprises reinjecting produced CO.sub.2 -hydrocarbon gas into the formation in a manner that more effectively utilizes the CO.sub.2 while maintaining most of the formation under miscible conditions thereby lowering the cost of oil recovery.